Sure. Zuck realized he wanted to do a lot of good with his money. If he thought that Facebook was the best way to have an impact, then he'd simply donate his shares back to the company, which would give them ~$45bn in capital to reinvest in their mission.

He didn't do that. Which means, as the person with the single greatest understanding of Facebook, he thought he could have a more positive impact on humanity by putting that money elsewhere.

This doesn't mean Facebook doesn't do good things. This doesn't mean Facebook isn't a great company. His actions merely show that, for some objectives, he thinks other organizations can have a better impact.